This invention relates generally to foam materials. More particularly, this invention pertains to expanded polymeric foam materials which include plant-based materials to enhance biodegradability.
Expanded foam materials, such as friable cereals, have been produced for many years within the food industry. Recently, this technology has been applied to lightweight resilient packaging materials utilizing grain based ingredients. In U.S. Pat. No. 4,282,121, for example, a starch-hydrolyzed polyacrylonitrile graft copolymer was dispersed in a polyhydric alcohol such as glycerol to form a resilient rubbery material. The degree of expansion was disclosed to be relatively small, i.e. 10-20 percent, indicating a relatively heavy foam material.
Expanded foam technology has been used in the plastics industry, particularly for manufacturing foamed packing materials with a twin-screw extruder. Use of single screw extruders has not been contemplated in such applications because of difficulties in achieving and controlling the desired degree of integral mixing and comminution.
Single-screw extruders pressurize and melt the materials by passage thereof between the threads of a single screw. In a twin-screw extruder, the two screws are overlapping to provide a higher degree of internal mixing and higher shear forces. Twin-screw extruders provide additional degrees of freedom in process control, and their use is dictated in other expansion processes.
British Patent Specification No. 1.485.833 has disclosed that plastics with carbon-carbon bonds can be rendered biodegradable by an addition of (a) starch or chemically modified starch and (b) an oxidizable substance such as a fatty acid and/or a fatty acid ester. The process has been found to be dependent upon transition metal content of the soil, and thus not generally applicable. The formation of starch-graft copolymers is not in view.
U.S. Pat. No. 4,931,488 of Chiquet discloses a method for enhancing the biodegradability of plastics by adding to the plastic (a) a biodegradable substance such as starch, (b) an iron compound, and (c) a fatty acid or fatty acid ester. Starch-graft copolymers are not in view, and the application to foamed products is not mentioned.
The grafting of saponified polyacrylonitrile to gelatinized starch is known, for example in U.S. Pat. Nos. 3,981,100, 3,985,616 and 3,997,484 of Weaver et al., resulting in products of very high water absorptive capabilities. The starch-graft polymers formed contain 60-90% acrylonitrile resin.
U.S. Pat. No. 3,138,564 of Borunsky et al. teaches that monomers such as acrylonitrile may be grafted to an oxidized or ozonized-oxidized starch.
EPO Patent Publication No. 0 118 240 of Wittwer et al. discloses the "destructurizing" of starch by treatment at supra-melting temperatures and low shear pressures in the presence of water. The starch structure is changed to form a gel under these mild conditions, improving its use in mold-forming gel capsules therefrom.
U.S. Pat. No. 4,026,849 of Bagley et al. discloses a method for producing a biodegradable filled composite plastic by grafting a high percentage, i.e. 40-60%, of a thermoplastic monomer onto starch. The product exhibited little or no die swell and could be extruded at low moisture content to make a strong insoluble plastic of good quality. Starch-graft copolymers prepared in accordance with the method disclosed by Bagley et al. are produced under conditions of higher temperature, i.e. more severe than disclosed in the Wittwer et al. patent, and are useful in the practice of the present invention. The use of the starch-graft copolymer in expanded products is not in view in the Bagley et al. patent.
Much effort has been expended to find biodegradable polymer containing materials. It has been found that many so-called biodegradable materials are biodegradable over time under laboratory conditions, but remain undegraded for long periods in the natural environment. A major reason for this phenomenon is because the articles do not rapidly disintegrate into small particles which become integrated with a soil layer containing the required constituents for disintegration, e.g. moisture, microorganisms, and growth components such as ammonia nitrogen.
Another reason is that the disintegration of some "biodegradable" materials is dependent upon solar radiation. Burial in a landfill effectively cuts off exposure to the sun, inhibiting degradation.